1. Field of the Invention
The present invention relates to a color image processing apparatus for digital color image processing.
2. Description of the Prior Art
In the field of non-silver color copiers, there have been proposed the following processes:
(1) an electrophotographic process in which the original image is separated through color separation filters to form latent images corresponding to different colors, which are respectively developed into visible images of three different colors and are transferred in registration to reproduce the original image;
(2) an ink jet recording process in which the original is scanned with color separation to obtain color separated signals, which are used, after electric conversion to complimentary colors and eventual color correction, for emitting ink of three or four colors from ink jet nozzles thereby reproducing the color image; and
(3) a thermal transfer recording process in which the original is scanned with color separation to transfer color inks through one or more thermal heads thereby reproducing the color image.
The above-mentioned process (1) relies on the analog characteristic of the electrophotographic process for the reproduction of intermediate tones required in the color images and is therefore associated with a considerable fluctuation in the image quality due to circumferential conditions. Such fluctuation is believed to be caused by the direct effect of temperature and humidity on the corona discharge, photosensitive member, etc.
On the other hand, the above-mentioned processes (2) and (3) involve various problems yet to be solved in relation to recording reliability printing speed, printing dot quality, etc.
FIG. 1 shows a conventional color copier manufactured by the present applicant, wherein a drum 1, rotated in a direction of arrow a, is provided, along the periphery thereof, with a photosensitive member composed of a conductive layer, a DcS photoconductive layer and an insulating layer.
An original carriage glass 3 supports an original to be copied. Said original is illuminated by an illuminating lamp 5, and the reflected light is scanned by scanning mirrors 7, 9 which are moved in synchronization with the rotation of the drum 1, and is guided through a lens 11, a mirror 13, a color separator 15, a mirror 17 and a secondary charger 19 for charge elimination simultaneously with exposure to light and is focused onto the photosensitive member of the drum 1.
In this manner a latent image is formed on the photosensitive drum 1.
The color separator 15 comprises a blue filter 15B, a green filter 15G, a red filter 15R and a neutral density (ND) filter 15N, which are suitably changed by rotation to achieve color separation.
The photosensitive member of the drum 1 is cleaned in advance with a blade cleaner 31, and the effect of previous latent image formation is erased by a pre-exposure lamp 33 and a pre-charger 35.
Then, the photosensitive member is uniformly charged with a primary charger 37 to obtain a uniform surface potential. Subsequently the photosensitive member is subjected to charge elimination by the secondary charger 19 simultaneously with exposure to the light from the original, and is then exposed uniformly to the light from a flush exposure lamp 39, thereby forming an electrostatic latent image of an elevated contrast on said photosensitive member.
In the vicinity of the drum 1 and between the flush exposure lamp 39 and a developing station 41 there is provided a potential probe 43 for detecting the electrostatic potential, or the intensity of the latent image.
The developing station 41 is composed of a yellow developing unit 41Y, a magenta developing unit 41M, a cyan developing unit 41C and a black developing unit 41B, which develop the latent image with toners of respective colors.
A recording sheet 51 stored in a cassette is supplied by a feeding roller 53 to a transfer station 55, where the sheet 51 is gripped at the leading end by a gripper 57 and the developed image on the photosensitive member of the drum 1 is transferred onto said sheet by means of corona discharge applied on the opposite face of said sheet from a transfer charger 59.
In the case of single-color copying, the recording sheet 51 is separated from the transfer station 55 by a separating claw 63 after the charge is eliminated by a separating charge eliminater 61.
On the other hand, in case of multi-color copying, the gripper 57 of the transfer station 55 is not released and the separating claw 63 does not operate so that the recording sheet 51 is retained, until the transfer of plural color images is completed.
Upon completion of the transfer, the separating claw 63 is activated to separate the recording sheet 51 from the transfer station 55, and said sheet is forwarded by a conveyor belt 65 to a fixing station 67 with heating rollers for image fixation.
The recording sheet 51 after image fixation is discharged into a tray 69. On the other hand, the drum 1 is cleaned, after image transfer, with the blade cleaner 31 for removing the remaining toner, and enters the succeeding copying cycle.
In the above-described structure in which the steps of original reading to latent image formation is conducted through a two-dimensional optical system, it is not possible to apply a particular process to each dot of the image, for example an imaging process such as masking to each dot in relation to the spectral characteristic of toners. For this reason the image quality of the reproduced color image is limited.
Also the reproduction of intermediate tones, indispensable in colored images, is achieved by regulating the surface potential on the photosensitive member in response to the intensity of light reflected from the original, thus modulating the amount of toner deposition in the developing step.
Consequently the image quality is apt to fluctuate, depending on the circumferential conditions.
There is also known a process in which the light reflected from an original is read, after color separation, with an image sensor such as a charge-coupled device (CCD), and the image is reproduced with suitable recording means such as lasers after performing determined image processing.
In such image processing, in order to control the deposition of toners of three or four colors in consideration of the entered color components of red, green and blue and of the colors of the toners to be employed, it is essential to know the magnitude of three color signals of each dot on the original.
For this purpose, there may be considered a structure of for scanning the original three times with color separation to store the image signals of different colors in an image memory and reading said image signals thereafter to calculate the amount of toners. Such structure not only requires a longer time for color separation, but also is disadvantageous if an image memory of high cost is employed, since, in case of copying an original of A3 size with a resolution of 10 lines/mm, there will be required a memory of 394 mm.times.420 mm.times.10 lines/mm.times.10 lines/mm.times.3 colors.times.6 bits/dot=2979 Mbits=37.2 Mbytes.
The memory capacity may be reduced by recording images of different colors simultaneously on different drums, but such structure will inevitably lead to a bulky and complicated apparatus since three to four drums are required for a full-color copying.